Motor vehicle air conditioning circuit provided with pre-expansion device

ABSTRACT

The invention is directed to an air conditioning unit having a refrigerating fluid loop comprising a compressor, a condenser, a liquid/gas separating reservoir, an expansion device, an evaporator and a pre-expansion device, inserted between the condenser and the reservoir, which is capable of producing a pressure drop ranging between 1.5 and 14 bars to reduce the fluid pressure down to its vapor saturating pressure. The reservoir can also contain a variable amount of liquid to compensate the fluid losses in the loop by maintaining the sub-cooling temperature constant in the condenser and consequently the heating capacity capable of being absorbed by the evaporator.

BACKGROUND OF THE INVENTION

The invention relates to a liquid coolant loop, in particular for an airconditioning installation for the passenger compartment of a vehicle.

JP-A-95 81383 describes such a loop comprising a compressor capable ofraising the pressure of the coolant in the gaseous state, a condensercapable of condensing the coolant compressed by the compressor and ofundercooling it to the liquid state, a preliminary pressure reductiondevice capable of lowering the pressure of the coolant coming out of thevessel and an evaporator capable of making the coolant coming from thepressure reducing valve pass from the liquid state to the gaseous statebefore its return to the compressor.

FIG. 1 is a diagram representing a thermodynamic cycle described by theliquid coolant in an air conditioning loop, traced in a system ofenthalpy/pressure coordinates. In this system, a bell-shaped curve Lenvelops a zone of coexistence between liquid and gas, whereas thecoolant is entirely in the liquid state to the left of the left-handside of the curve and entirely in the gaseous state to the right of theright-hand side.

The cycle substantially has the shape of a rectangular trapezium withhorizonal bases. From a point A situated in the gaseous zone, thecompressor conveys the coolant in the gaseous state to a point Bcorresponding to a higher enthalpy and pressure than at point A. In thecondenser, the coolant passes through a horizontal segment from point Bto a point E situated in the liquid zone, which segment passes throughthe right-hand and left-hand sides of the curve L at points C and Drespectively. Segments BC, CD and DE correspond respectively to adesuperheating of the gaseous coolant, to the condensation and anunder-cooling of the coolant in the liquid state. At the entrance of theevaporator, the coolant is at a point G situated in the liquid/gas zone,corresponding to the same enthalpy value as point E and to the samepressure value as point A. In the evaporator, the coolant is returned topoint A by passing through, at H, the right-hand side of the curve L.

In conventional liquid coolant loops, the coolant passes through theseparating vessel at point E of the thermodynamic cycle, and passesthrough segment EG in the pressure reducing valve. As point E issituated in the liquid zone, the vessel is then completely filled withliquid and the quantity of coolant which it contains cannot vary. Whenthe total mass of the liquid coolant contained in the loop falls,especially by virtue of leaks in the circuit, this reduction isperformed in particular at the expense of the condenser, theundercooling capacity of which is thus reduced, which has the effect ofraising the enthalpy level of the coolant at the outlet of the condenserand at the inlet of the evaporator and consequently of reducing theuseful heat absorbed by the coolant in the evaporator.

One solution to this problem lies in departing from the conventionalarchitecture by positioning the separating vessel between a condensationpart and an undercooling part of the condenser, so that thethermodynamic state of the coolant in the vessel corresponds to point Dof the cycle, situated on the saturation curve, which allows the vesselto contain a quantity of coolant that can vary as a function of thetotal mass of coolant in the circuit.

BRIEF SUMMARY OF THE INVENTION

According to the invention, this same result may be obtained in a loopsuch as that defined in the introduction, by positioning between thecondenser and the vessel a preliminary pressure reduction device whichis capable of producing a pressure loss of between 1.5 and 14 bars so asto bring the pressure of the coolant back up to its saturated vapourpressure.

The preliminary pressure reduction device conveys the fluid from thethermodynamic state corresponding to the point E to that correspondingto the point F, situated again on the saturation curve, in which statethe coolant continued in the separating vessel is consequently situated.The pressure reducing valve then conveys the coolant from point F topoint G.

It is also known from JP-A-93223365 to place a device that produces apressure loss between the condenser and the vessel. However, from thisdocument it does not follow that this pressure loss brings the pressureof the coolant back again to its saturated vapour pressure. Furthermore,the only values disclosed for the loss of pressure are 0.5 and 1.0kg/cm², the latter value bringing about a substantial loss in coolingcapacity of the loop.

Complementary or alternative optional characteristics of the inventionare given below:

The preliminary pressure reduction device is capable of producing apressure loss of between 4 and 10 bars.

The preliminary pressure reduction device comprises a constrictiondefining a minimum passage section of between 0.2 and 7 mm²approximately in a pipe through which the entire flow of coolant leavingthe condenser passes.

The passage section substantially retains its minimum value over alength of between 0.1 and 5 mm.

The minimum passage section does not exceed 50% of the passage sectionof the pipe upstream and/or downstream from the constriction.

The passage section reduces progressively in an initial region of theconstriction, substantially retains its minimum value in an intermediateregion and progressively increases in a final region.

The constriction is formed by an insert introduced into a substantiallycylindrical pipe.

The constriction is formed by a thickening of the wall of asubstantially cylindrical pipe.

The constricted passage is adjacent to the cylindrical wall.

The constricted passage is substantially centred in relation to thecylindrical wall.

The constriction is formed by a washer lock-beaded into the pipe.

The preliminary pressure reduction device is housed in an outlet pipemounted on a manifold of the condenser.

The said outlet pipe is contained in the separating vessel anddischarges into a gas collection space thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics and advantages of the invention will be shown infurther detail in the following description, with reference to theattached drawings, on which:

FIG. 1 is a thermodynamic diagram which has already been commented onabove;

FIG. 2 is a diagram showing the variation of the degree of undercoolingproduced by the condenser as a function of the mass of coolant in a loopaccording to the invention;

FIG. 3 is a diagram of a liquid coolant loop according to the invention;

FIG. 4 is a diagram showing the variation of the refrigerating capacityof a loop according to the invention as a function of the pressure lossproduced by the preliminary pressure reduction device;

FIG. 5 is a diagrammatical representation of a condenser having anoutlet manifold and an outlet pipe, either the one or the other of whichmay receive a preliminary pressure reduction device according to theinvention;

FIGS. 6 to 8 are diagrammatical representations showing different waysof producing a constriction in the outlet pipe; and

FIG. 9 is a diagrammatical representation of a single-piece sub-assemblycomprising a condenser, a preliminary pressure reduction device and aseparating vessel.

DETAILED DESCRIPTION OF THE INVENTION

The loop 1 shown diagrammatically on FIG. 3 comprises a compressor 2, acondenser 3, a preliminary pressure reduction device 4, a separatingvessel or “cylinder” 5, a pressure reducing valve 6 and an evaporator 7through which the liquid coolant passes in this order. The lower part ofthe vessel 5 is filled with coolant in the liquid state, the residualgas which penetrates into the vessel remains above the liquid level andonly coolant in the liquid state is taken off beneath this level to besent towards the pressure reducing valve 6. Diagrammatically indicatedin the condenser 3 are a desuperheating portion 3-1 where the coolant inthe gaseous state coming from the compressor is cooled to a liquid-gasequilibrium temperature, a condensation portion 3-2 where the coolant iscondensed to the equilibrium temperature, and an undercooling portion3-3 where the coolant in the liquid state is cooled beneath theequilibrium temperature. Similarly, the evaporator comprises avaporisation portion 7-1 and a superheating portion 7-2.

FIG. 2 provides a curve that is representative of the variation of thedifference ΔT between the liquid/gas equilibrium temperature in thecondenser (condensation temperature) and the temperature of the coolantat the outlet of the condenser, after undercooling, as a function of themass m of fluid contained in a loop according to the invention. Thiscurve is formed of a first portion rising to a value m₁, a secondhorizontal portion from m₁ to m₂ and a third portion rising to beyondm₂. The plateau is obtained thanks to the variation in the quantity ofcoolant contained in the vessel 5, the values m₁ and m₂ correspondingrespectively to the minimum and maximum liquid levels therein. Thedegree of undercooling, and consequently the performances of the loop,remain substantially constant until the leakages return the mass ofcoolant to m₁. The initial mass of coolant is preferably chosen in thevicinity of m₂ so that the stable operating time is as long as possible.The length of the plateau is itself a function of the liquid-gasseparation capacity and of the volume of the vessel.

For the conventional loop mentioned above, in which the thermodynamicstate of the fluid in the separating vessel corresponds to point E ofthe cycle, the plateau of the curve of FIG. 2 does not exist and thedegree of undercooling varies continuously with the quantity of coolant.

In the diagram of FIG. 3, the preliminary pressure reduction device isrepresented by way of example in the form of a diaphragm 4-1 disposed atright angles to the path of the coolant and having an orifice 4-2. Asshown, the condenser 3, the preliminary pressure reduction device 4 andthe vessel 5 may be disposed spaced apart and interconnected byconnecting pipes.

The preliminary pressure reduction device, regardless of its shape andposition, improves the refrigerating capacity provided by the airconditioning loop, as the curve of FIG. 4 shows, which represents thevariation of this capacity as a function of the loss of pressureproduced by the preliminary pressure reduction device. It is noted thatthe refrigerating capacity exceeds a maximum for a loss of pressure of 9bars, an improvement being observed, in relation to the absence of thepreliminary pressure reduction device, at least in the range of 1.5 to14 bars, and more particularly in the range of 4 to 10 bars.

FIG. 5 diagrammatically shows a condenser comprising an inlet manifold10 equipped with an inlet pipe 11, an outlet manifold 12 equipped with aoutlet pipe 13 and a bank of tubes 14 through which the coolantcirculates between different chambers formed in the manifolds. Thepreliminary pressure reduction device of the invention mayadvantageously be housed either in the manifold 12, or in the pipe 13,in the zone A where they are connected.

FIG. 6 shows a preliminary pressure reduction device formed by an insert20 introduced into the outlet pipe 13 and applied against thecylindrical wall thereof, over a fraction of its perimeter, leaving aconstricted passage 21 adjacent to the remaining portion of theperimeter of the wall. The passage 21 has a minimum passage section S2which does not exceed 50% of the passage section S1 of the pipe 13upstream and downstream from the insert 20, the section S2 beingadvantageously being 0.2 and 7 mm² approximately. In the plane ofprojection, the insert 20 has a trapezoidal profile thanks to which thepassage section progressively reduces in an initial region of theconstriction, retains its minimum value in an intermediate region andprogressively increases in a final region.

In the variant of FIG. 7, the insert 20 is replaced by a washer 22disposed at right angles to the pipe 13 and having a central hole 23defining a constricted passage, the passage section of which is constanthere and meets the conditions mentioned with respect to the minimumpassage section of the constriction 21. The length of the constriction23, i.e. the thickness of the washer 22, is between 0.1 and 5 mm. Thewasher 22 may be fixed by lock beading, deforming the thin wall of thepipe 13 on either side of its thickness.

FIG. 8 shows an outlet pipe 13 having a thick wall, which for example ismoulded, fixed mechanically to the manifold 12 of the condenser. Thepipe 13 has an internal circumferential rib 24 which leaves a centralpassage of reduction section. The rib 24 has a trapezoidal profile,thanks to which the passage section of the constriction 25 varies in asimilar manner to that of the constriction 21.

FIG. 9 shows a condenser 3 similar to that of FIG. 5 and a separatingvessel 5 coupled to the side of the outlet manifold 12, the manifold 12and the vessel 5 being vertically lengthened. The outlet pipe 13 ishoused in the vessel 5 and curves so as to extend upwards and open intothe upper portion of the vessel, in a space 30 where the residual gasleaving the condenser collects. The space 30 is separated from the lowerspace 31 of the vessel by a filtration zone 32 through which the pipe 13passes. The coolant in the liquid state leaves the vessel by a lowerpipe 33 communicating with the space 31. The preliminary pressurereduction device 4 is interposed on the pipe 13 and consequently housedin the vessel 5.

What is claimed is:
 1. A liquid coolant loop for an air conditioninginstallation for the passenger compartment of a vehicle, comprising acompressor capable of raising the pressure of the coolant in the gaseousstate, a condenser capable of condensing the coolant compressed by thecompressor and undercooling it to the liquid state, a preliminarypressure reduction device capable of lowering the pressure of thecoolant leaving the condenser, before its passage into a separatingvessel capable of separating residual gas of the coolant in the liquidstate coming from the condenser, a pressure reducing valve capable oflowering the pressure of the coolant leaving the vessel and anevaporator capable of making the coolant coming from the pressurereducing valve pass from the liquid state to the gaseous state beforeits return to the compressor, wherein the preliminary pressure reducingdevice is capable of producing a pressure loss of between 1.5 and 14bars so as to bring the pressure of the coolant back to its saturatedvapour pressure.
 2. A loop according to claim 1, wherein the preliminarypressure reduction device is capable of producing a pressure loss ofbetween 4 and 10 bars.
 3. A loop according to claim 1, wherein thepreliminary pressure reduction device comprises a constriction defininga minimum passage section of between 0.2 and 7 mm² approximately in apipe through which the entire flow leaving the condenser passes.
 4. Aloop according to claim 3, wherein the passage section substantiallyretains a minimum value over a length of between 0.1 and 5 mm.
 5. A loopaccording to claim 4, wherein the minimum passage section does notexceed 50% of the passage section of the pipe upstream and/or downstreamfrom the constriction.
 6. A loop according to claim 3, wherein thepassage section progressively decreases in an initial region of theconstriction, substantially retains its minimum value in an intermediateregion and progressively increases in a final region.
 7. A loopaccording to claim 3, wherein the constriction is formed by an insertintroduced into a substantially cylindrical pipe.
 8. A loop according toclaim 3, wherein the constriction is formed by a thickening of a wall ofa substantially cylindrical pipe.
 9. A loop according to claim 7,wherein the constricted passage is adjacent to the cylindrical wall. 10.A loop according to claim 7, wherein the constricted passage issubstantially centred in relation to the cylindrical wall.
 11. A loopaccording to claim 10, wherein the constriction is formed by a washerlock-beaded into the pipe.
 12. A loop according to claim 1, wherein thepreliminary pressure reduction device is housed in an outlet pipemounted on a manifold of the condenser.
 13. A loop according to claim12, wherein the outlet pipe is contained in the separating vessel anddischarges into a gas collection space thereof.